The unfortunate occurrence when a prison inmate drops the soap in the shower. As they bend down to reach for the soap, they are forcibly penetrated in the ass by a horny, desperate inmate behind them. Thats why you shouldn't go to jail, kiddies!

Inmate: Ooops! I dropped the soap! I sure hope a big, strong, black man doesn't do me up the butt! Teehee!
*Bends down to grab the soap in a seductive manner and gets buttsexed*
Inmate: mmmm, dropping the soap is so fun!

(Proverbial Meaning:)
1) Having a lot of sexual intercourse with a lot of different persons.
2) Being situated in a risky situation, where one mistake can be fatal.
(Literal Meanig:)
Dropping the soap in prison. When you drop the soap in prison and you pick it up, you're bending over, which is the perfect oppertunity for your fellow prisoners to take you from behind and penetrate you in an unpleasant way.

(Proverbial Meaning:)
1) You're dropping the soap a lot lately, stop being so slutty!
2) Please help me! If i drop the soap now, im screwed!